In an optical communication system, it is frequently necessary to couple a first optical fiber to a second optical fiber. These optical connections or junctions have been achieved with adapters that receive and engage standard connector pairs. In these single fiber connectors, the respective fibers are usually terminated in a ferrule. The ends of the respective fibers are typically cleaned before being arranged in a sleeve within the ferrule. The corresponding sleeves are precisely arranged to guide the ends of the fibers into alignment. Typically, the ferrules are configured to apply a biasing force against respective surfaces of the sleeves to ensure that the ends of the fibers remain in contact. When properly cleaned and arranged, the optical junction achieves acceptable performance levels.
Optical junction quality becomes more important with increasing optical signal data rates. Optical junction quality is defined by insertion loss (attenuation) and return loss (reflection). Insertion loss is the difference in signal power resulting from the optical junction. Return loss is the ratio of incident power to reflected power at the optical junction. Measurements of these parameters are now defined in International Electrotechnical Commission (IEC) standard 61753-1. The standard gives five grades for insertion loss from A (best) to D (worst), and M for multimode. The standard further provides five grades for return loss, with grades from 1 (best) to 5 (worst). Many industry standards include strict limits on return loss and insertion loss introduced by fiber connections.
The results are quite different for multi-fiber push on (MPO) optical junctions. MPO optical junctions include an array of optical fibers precisely located at the end of a ferrule molded from a plastic material. The end of the ferrule is generally polished and the ferrule is arranged to enable a short length of the individual fibers to protrude beyond the face of the ferrule. The MPO optical junctions are completed with male and female connectors. The male version of the connector is arranged with guide pins that extend from the ferrule of the connector. The female version is arranged with alignment holes that closely receive the guide pins. The resulting glass-to-glass contact eliminates signal losses that would be caused by an air gap between the joined fibers. While this arrangement can achieve fiber to fiber alignment and contact there are a number of factors that can prevent the intended alignment and contact between each of the corresponding ends of the optical fibers in a multiple fiber optical junction.
With proper fiber to fiber alignment and contact a return loss of 20 dB (a ratio of power sent over power returned) is easily attainable. In traditional optical networks, an allowable return loss from the entire link is limited to 12 dB minimum. Return losses of less than 12 dB may enhance harmonic modulation distortion and increase noise in the laser light source, which can cause instability and introduce data errors in optical signals. A single glass to air interface produces 14 dB of return loss. A single improper fiber to fiber connection has two glass to air interfaces, and can introduce return loss as small as 10 dB. In addition, an improper connection can introduce interference between a reflected signal and an intended signal which can lead to unpredictable results.
Improper connections can result from a number of factors. For example, manufacturing tolerances in the arrangement of the optical fiber ends in a connector can introduce one or more offsets from a desired arrangement. By way of further example, variations between the polished surface of the ferrule and the internal sleeves or other structures supporting the fibers can lead to unintended variations in the length of the exposed fibers. This can also occur when a polishing step has produced a ferrule that is not flat across the mating surface. In addition, manufacturing tolerances in the size, shape and location of the guide pins and their corresponding holes or recesses in a female connector can lead to misalignment between the corresponding fibers when the ferrules are coupled. Moreover, particles of ferrule material or foreign material can remain on the ferrule surface. While it is standard practice to clean the ferrule surface it is sometimes difficult to remove dirt near the base of the alignment pins. Any of the mentioned factors can prevent a consistent fiber to fiber contact for one or more of the optical junctions.
A conventional (male) ferrule with guide pins is illustrated in a top plan view and a side elevation view in FIG. 1A. The conventional ferrule 1 includes a base 3 with guide pins 2 extending beyond a polished face 5 of the base 3. Optical fibers 4 traverse the base 3 and extend beyond the polished surface 5 about 1 to 5 μm. A conventional (female) ferrule with recesses arranged to receive the guide pins 2 is illustrated in a top plan view and a side elevation view in FIG. 1B. The conventional ferrule 10 includes a base 13 with recesses 12 extending from an opening in a polished face 15 into the base 13. Optical fibers 14 traverse the base 13 and extend beyond the polished surface 15 about 1 to 5 μm.
FIG. 2 illustrates an improper arrangement of the ferrule 1 with the ferrule 10 introduced in FIGS. 1A and 1B in a side elevation view. As shown in FIG. 2, when the conventional ferrule 1 is in registration and close arrangement with the ferrule 10, i.e., when the guide pins 2 of the ferrule 1 are inserted in the corresponding recesses 12 in the ferrule 10, debris 20 having a dimension greater than about 2-10 μm between the polished face 5 and the polished face 15 results in an unintentional gap 25 between optical fiber 4 and optical fiber 14. The resulting gap introduces an abrupt change in the refractive index in the signal path supported by the respective optical fiber pairs. Such abrupt changes in the refractive index of the light path generates a reflection R defined by equation 1 below:
                    R        =                                            (                                                n                  1                                -                                  n                  g                                            )                        2                                              (                                                n                  1                                +                                  n                  g                                            )                        2                                              Equation        ⁢                                  ⁢                  (          1          )                    where, n1 is the refractive index of the material used to manufacture the optical fiber and ng is the refractive index of air. The reflected light decreases the amount of transmitted light. Since the gap introduces reflective interfaces at the surface of each fiber, the minimum loss to the transmitted light T in each of the separate light paths is defined by equation 2.
                    T        =                              1            -                          2              ⁢              R                                =                      1            -                                          2                ⁢                                                      (                                                                  n                        1                                            -                                              n                        g                                                              )                                    2                                                                              (                                                            n                      1                                        +                                          n                      g                                                        )                                2                                                                        Equation        ⁢                                  ⁢                  (          2          )                    
Insertion loss, which is additive to the loss in transmitted light due to the reflections, is dependent on the distance of separation between the corresponding fiber pairs.
Various tools have been developed in an attempt to address cleanliness at the termination of an optical fiber. For example, U.S. Pat. No. 5,956,793 discloses an optical fiber cleaning tool that includes a strip of adhesive tape within a housing of the tool. The tool includes a latch for releasably attaching the tool to a connector with installed optical fibers. While an adhesive tape is somewhat effective for capturing undesired debris, any adhesive that adheres to the ferrule or ferrules that is not removed by a secondary cleaning step can lead to other debris becoming attached to surfaces that can lead to gaps between fiber pairs.
U.S. Patent Application Publication No. 2003/0039463 discloses a tool for cleaning a single fiber optical connector. The tool includes a cleaning sheet in a housing and a guide member that directs the single fiber optical connector at an angle such that the exterior surface of the fiber is parallel to the cleaning sheet when the connector is inserted in the tool.
In many data centers, optical networks are supported by trunks of parallel fibers with MPO optical junctions. A typical communication link between server computers may be supported by two or more MPO optical junctions. The conventional connection systems, cleaning tools, and commercial systems for quickly terminating a ribbon fiber do not address the above-described issues which prevent achievement of consistent optical coupling between multiple optical fibers at an optical junction. Thus, as the number of such multiple fiber optical junctions increases in a communication link, signal integrity within the network supported by such links is at increased risk.